35mm format transparencies

ABSTRACT

A 35 mm format transparency comprising a dye diffusion thermal printing receiver sheet consisting of a transparent substrate having on one surface thereof an image layer on which the image is formed by thermal dye transfer using a laser as the source of heat to effect the thermal transfer, the transparent substrate and the image layer consisting of materials having, in combination, a moisture absorption of less than 1.5% and being positioned removably between two transparent glass sheets.

This invention relates to 35 mm format "transparencies" which can, inwell known manner, be used in a projector to provide a magnified versionof an image stored on the transparency.

Conventionally such transparencies are produced photographically andhence consist of a transparent substrate having thereon a gelatincoating containing light sensitive chemicals in which the image isformed by exposure to light in a camera and then treatment by suitabledeveloping techniques.

Such transparencies are relatively delicate physically and are easilyscratched. It is common practice, therefore, to place the actualtransparency between two sheets of glass for protection.

However, under certain conditions, this can produce a disadvantageousside effect.

Both gelatin and cellulose triacetate (the base film most commonly usedfor photographic film) are hygroscopic and, therefore, have anequilibrium water content which depends on the relative humidity and thetemperature at which they are stored. (Gelatin/silver halide emulsioncombinations are about ten times more hygroscopic than cellulosetriacetate). What is more, the level of moisture in a photographic filmneeds to be controlled to give optimum film properties. Levels that aretoo low lead to problems with film brittleness, curl, static anddimensional changes and levels that are too high lead to tackiness ofthe film and can lead to ferrotyping problems.

When the transparency is placed in a projector a portion of the lighttransmitted through the transparency to form the magnified image isabsorbed by the transparency image. The transparency, therefore isheated more rapidly than the glass and a temperature gradient isproduced at the interface between the transparency and the glass. Themoisture present in the gelatin can condense out and cause fogging ordistortion of the projected image until temperature equilibrium betweenthe transparency and the glass is reached. Clearly, the greater theinitial temperature gradient, the more pronounced the effect is likelyto be.

It is an object of this invention to obviate this effect.

According to one aspect of the invention, there is provided a 35 mmformat transparency comprising a transparent substrate having an imageon one surface thereof and positioned between two transparent protectivelayers, characterised in that the transparent substrate and the imagelayer consist of materials having, in combination, a moisture absorptionof less than 1.5%

Preferably, the moisture absorption is less than 1%.

According to a preferred aspect of the invention, the transparentsubstrate and image layer together comprise a dye diffusion thermaltransfer receiver sheet, in which the image is formed by dye diffusionthermal transfer techniques.

Dye diffusion thermal transfer is a generic term for processes in whichone or more thermally transferable dyes are caused to transfer from adye sheet to a receiver sheet in response to thermal stimuli. Using adye sheet comprising a thin substrate supporting a dye coat containingone or more such dyes uniformly spread over an entire printing area ofthe dye sheet, image production can be effected by heating selecteddiscrete areas of the dye sheet while the dye sheet is in contact with areceiver sheet, thereby causing dye to transfer to corresponding areasof that receiver sheet. Full colour, high resolution photographicquality images can be produced by using different coloured dye coatssequentially in like manner and appropriate equipment, such as aprogammable thermal print head or laser controlled by electronic signalsderived from, for example a video camera or similar signal generatingapparatus. The resolution is particularly high if a laser is used and inpractice such a heat source is preferred for 35 mm transparencypreparation.

A typical receiver sheet consists of a substrate, supporting a receivercoat of a dye receptive composition containing a material highlyreceptive to the dyes being transferred and containing additives topromote the release of the dye sheet from the receiver sheet afterimaging has taken place. For use in 35 mm format transparencies, thereceiver sheet must, of course, be transparent.

Substrate materials having a moisture absorption of less than 1% andwhich are preferred for use in the present invention are polyesters suchas polyethylene terephthalate, polyethylene aphthanate or polybutyleneterephthalate or polyolefins such as polypropylene. Alternatively,polycarbonate or polyvinyl chloride may be used.

The receiver coat may be formed from any material which is solventcoatable, compatible with the dyes being used and has a moistureabsorption of less than 1%. Particularly suitable are solvent solublepolyesters, for example, Vylon polyesters made by Toyosho, but polyvinylchloride and styrene-acrylonitrile copolymer may also be used. Thesematerials may be used alone or as mixtures.

The invention will be more readily understood from the followingexamples.

EXAMPLE 1

An electronically stored image was recorded as a 35 mm transparency onto standard E6 Kodak photographic film using a Polaroid C15000 printer.The same image was then recorded on to a laser thermal transfer receiversheet by holding the receiver sheet together with three separate dyesheets (comprising coatings of thermally transferable dye and polymericbinder on 23 μm Melinex, the three dye sheets having yellow, magenta andcyan dyes respectively) by the application of one atmosphere pressure. ASpectroDiode Laboratories 150 mw diode laser operating at 807 nm wasfocussed down into the dye sheet/receiver sheet combination and scannedacross so as to build up the appropriate dye transfer.

The receiver sheet consisted of a transparent polyethylene terephthalate(125 μm Melinex from ICI) film coated with the following formulationfrom a 50/50 mixture of toluene/ethyl methyl ketone:

    ______________________________________                                        Vylon 200    30 parts Cymel 303     6 parts                                   Vylon 103    70 parts Tergomer HSi                                                                              0.7parts                                                          2210                                                    Ketjenflex MH                                                                             7.5 parts Tinuvin 900   1 part                                    R 4046      0.4 parts                                                         ______________________________________                                    

to a dry coat thickness of 3 μm which was cured at 140° C. for threeminutes. (Vylon 200 and Vylon 103 are polyesters having a high dyeaffinity sold by Toyobo; Tergomer HSi 2210 is a bis-hydroxyalkylpolydimethylsiloxane sold by Th Goldschmidt, Cymel 303 is ahexamethoxymethylmelamine oligomeric cross-linking agent from AmericanCyanamid, Ketjenflex MH is a toluene sulphonamide/formaldehydecondensate from Akzo, Tinuvin 900 is a UV light absorber sold byCiba-Geigy and R4046 is an amine blocked p-toluene sulphonic acidcatalyst).

Both transparencies were placed in protective glass mounts and, with themounts open were placed in a humidity ageing cabinet held at 45° C. and85% RH for 5 minutes. The transparencies were then removed, closed andallowed to equilibrate to ambient conditions. The images were thenprojected on to a white screen using a Kodak S V 2010 projector. After 5seconds the E6 image darkened due to condensation of water on the insideof the glass mount. The darkened areas gradually lightened as thecondensed water evaporated and 5 minutes elapsed before all the waterhad gone. No such darkening occurred with the thermal transfer image.The thermal transfer image was replaced in the humidity cabinet for afurther 72 hours and again tested in the projector. No condensation waspresent.

EXAMPLE 2

The moisture uptake of three film samples was determined by storing themin a humidity ageing cabinet held at 45° C. and 85% RH for 72 hours,accurately weighing them, transferring them to a vacuum oven held at120° C. and 1 cm of mercury for 3 hours and re-weighing them, the %moisture uptake being calculated from the difference between the dry and"wet" weights of the samples.

The samples tested were:

Sample 1. A roll of standard 35 mm film comprising a silver halideemulsion on cellulose triacetate base.

Sample 2. A sheet of exposed lithographic film comprising a silverhalide emulsion on a polyethylene terephthalate base.

Sample 3. A sheet of thermal transfer receiver sheet as described inExample 1.

The moisture uptake values obtained were as follows:

    ______________________________________                                        Sample      Moisture Uptake(%)                                                ______________________________________                                        1           3.81                                                              2           1.33                                                              3           0.57                                                              ______________________________________                                    

We claim:
 1. A 35 mm format transparency comprising a dye diffusionthermal printing receiver sheet consisting of a transparent substratehaving on one surface thereof an image layer on which the image isformed by thermal dye transfer using a laser as the source of heat toeffect the thermal transfer, the transparent substrate and the imagelayer consisting of materials having, in combination, a moistureabsorption of less than 1.5% and being positioned removably between twotransparent, glass sheets.
 2. A transparency according to claim 1, inwhich the moisture absorption is less than 1.0%.